Many ionic polymer membranes used in electrochemical cells as an electrolyte comprise only one active material. Inactive materials are often used as a scaffold and mechanical structure for the active polymer. Some such membranes are laminated, to provide differing properties at each electrode. In all cases, the properties and materials at each point on each of the two membrane electrode surfaces are the same.
Effective water management within an electrochemical cell is necessary, for good performance. For example, in a fuel cell, performance relies heavily on hydration control within the solid polymer electrolyte (SPE). During fuel cell operation, ions migrate through the SPE. As they do so, they take with them water molecules. This is known as electro-osmotic drag. The net result of this phenomenon, for example in a proton-conducting, cationic exchange (CE) SPE, is that the anode becomes prone to drying and the cathode to flooding. This impacts negatively on fuel cell performance, by reducing ease of ion conduction in the SPE and impeding oxygen access to the cathode. Since water is produced at the cathode as a result of the electrochemical reaction, the problem of cathode flooding is compounded. In a hydroxyl ion-conducting anionic exchange (AE) SPE, the situation is reversed, but the problems remain the same: water is produced at the anode, and used at the cathode, hence the anode is prone to flooding and the cathode is prone to drying.
Attempts to overcome these problems have been made via two principal routes. Firstly, the fuel supply has been humidified to help deliver water to the anode side of the SPE membrane, to reduce drying. This requires considerable balance of plant, which increases device cost and reduces power output, while introducing the risk of anode flooding. Secondly, thinner SPE membranes have been pursued, supposedly to reduce the distance water would have to back-migrate, from cathode to anode. However, electro-osmotic drag still restricts water equilibration via this mechanism, while resulting in mechanically less stable SPE membranes with higher fuel cross-over.
The voltage produced by a fuel cell generally depends on the electrochemical potential of the reactants, and the efficiency of any catalyst used. The current produced by the cell depends on its effective area, the ionic resistance of the membrane and how efficient the transfer of fuel is at the catalyst-membrane interface. The power output is a product of the voltage and current, and is normally controlled by throttling or restricting the rate of flow of fuel to the cell, or by providing the cell with unlimited fuel and controlling (e.g. by dumping to a battery) any energy surplus to requirements. These methods of controlling power output are undesirable, primarily for reasons of inefficiency.
WO03/023980 discloses hydrophilic polymers and their use in membrane electrodes assemblies (MEA's) that can be used in or as fuel cells.